The present invention relates to a screen printing machine in general, and more particularly to a screen printing machine having a rotary-jacket squeegee.
Screen printing machines known in the art either have endless printing screens which are of tubular configuration and which are rotatable, or they have belt-like printing screens which are trained about reversing rollers, or they have flat printing screens mounted in a frame. such machines are used for printing on textiles, paper, non-woven goods and the like.
All screen printing machines have in common that their printing screens are of a very thin material, for example a thin metal foil, a wire mesh or the like, having a very fine perforations therein. A selected printed pattern is produced on to these printing screens by making those portions of the printing screen which are not supposed to print on to a workpiece, inpenetrable to printing ink by coating them with lacquer or in any other manner known in the art. The downwardly facing surface of the printing screen is located above the path of the workpiece onto which it is intended to print. The upwardly facing surface of the printing screen has a printing medium admitted onto it, in the case of tubular rotary printing screens usually by means of an ink-discharging tube. The printing medium, such as ink, which is admitted onto the upper surface of the printing screen is squeezed through the not blocked perforations of the screen by means of a squeegee. Flat squeegees are known which are essentially strip-shaped and wipe over the surface of the printing screen to thus force the printing medium through the screen perforations. Another type of squeegee known in the art is a roller squeegee which is in contact or almost in contact with the upper surface of the printing screen so as to squeeze the printing medium through the perforations of the same. The present invention is particularly concerned with screen printing machines of the type having roller squeegees.
It is known from the prior art to provide a drive for roller squeegees by mounting a gear on the shaft of the squeegee and transmitting motion to the gear. The squeegee may be mounted so that it can be raised and lowered relative to the upper surface of the printing screen in order to be able to vary the gap between the upper printing screen surface and the circumference of the squeegee to thus be able to in turn vary the amount of pressure exerted by the squeegee upon the printing medium that is to be forced through the printing screen. The most common arrangement, however, is to have the squeegee rest under its own weight on the upper surface of the printing screen, i.e. in the case of a tubular printing screen on the inner circumferential surface of the same.
To obtain a uniform discharge of printing ink through the printing screen over the entire length of the roller squeegee, and thus to obtain a uniform printing upon a workpiece web, it is highly important to assure absolutely uniform contact of the squeegee roller periphery with the upper surface of the printing screen, i.e. the inner surface of a tubular printing screen. In cases where direct contact is not desired it is absolutely necessary that the gap defined between the roller squeegee periphery and the surface of the printing screen be uniform over the entire axial length of the roller squeegee.
This has not heretofore been possible.
If the roller squeegee rests under its own weight on the surface of the printing screen and pressure is exerted upon the opposite axial ends of the roller squeegee, this pressure does not cause the center region of the squeegee roller, as seen with reference to the opposite axial ends of the same, to be pressed correspondingly more firmly against the surface of the printing screen. Furthermore, the printing screen is of course itself in contact with the workpiece web, for example a carpet or the like that is to be printed and which has a not inconsiderable ability to yield to pressure by becoming compressed. This means that if, for example in the case of a tubular printing screen, pressure is exerted upon the axial ends of a roller squeegee which extends axially through this printing screen with the intention of pressing the roller squeegee more firmly against the inner circumferential surface of the printing screen, the printing screen is not sufficiently supported in the region of its own axial ends which usually coincide with the axial ends of the roller squeegee. The result of this is two-fold, in that downward pressure exerted upon the axial ends of the roller squeegee causes the center region of the squeegee to bow upwardly in direction away from the line of contact with the inner circumferential surface of the printing screen, and in that the rather fragile material of the printing screen itself will be formed with folds or wrinkles in the region of its axial ends where the roller squeegee end portions press firmly upon it, so that in a brief period of time the screen will become destroyed.
What this means is that in the prior art it has heretofore been impossible to exert upon the center region of the squeegee roller any type of pressure urging the same towards the line of contact with the printing screen surface in order to obtain a uniform contact with this surface or a uniform spacing of the squeegee roller periphery from this surface.